According to a conventional vehicular emergency reporting device, when a vehicle has a collision accident and the device detects a collision signal output from a collision sensor, the device transmits an emergency report signal to an external emergency facility for performing the emergency report function (refer to, for example, patent literature 1).
Generally, a vehicular emergency reporting device of above-described type includes a voltage step-down circuit that generates a decreased voltage by decreasing a main power source voltage supplied from a main power source (main battery), an auxiliary power source (auxiliary battery) functioning as a backup power source of the main power source, and a voltage step-up circuit that generates an increased voltage by increasing an auxiliary power source voltage supplied from the auxiliary power source. The voltage step-down circuit operates only in the case where the main power source voltage is supplied from the main power source. The voltage step-down circuit supplies the decreased voltage generated by decreasing the main power source voltage to a functional block, which cannot directly uses the main power source voltage as an operation voltage.
In the above configuration, the auxiliary power source is provided by a battery. Since the auxiliary power source needs to be embedded in a vehicular emergency reporting device, the auxiliary power source is requested to be downsized and the number of cells is requested to be reduced. However, the small number of cells inevitably causes a decrease in the auxiliary power source voltage supplied by the auxiliary power source decreases naturally. When the auxiliary power source is almost new and the operation voltage necessary for the emergency report is sufficiently assured by the auxiliary power source voltage, the emergency report can be carried out without any problem. However, the auxiliary power source has a characteristic that the internal resistance increases when the in-vehicle environment is under low temperature or deterioration occurs due to lapse of age of use, and the voltage value of the auxiliary power source voltage generally decreases. As a result, the operation voltage necessary to perform an emergency report may not be assured sufficiently by the auxiliary power source voltage. With consideration of such a circumstance, the voltage step-up circuit is provided to the device. The voltage step-up circuit increases the auxiliary power source voltage supplied from the auxiliary power source, and provides the increased voltage as the operation voltage for performing the emergency report
For example, when the main power source voltage is not supplied from the main power source due to breakage, disconnection, or the like of the main power source, the voltage step-up circuit continuously increases the auxiliary power source voltage supplied from the auxiliary power source. However, the increase of the voltage inevitably causes a loss in the voltage step-up circuit. Consequently, even when the voltage step-up circuit continuously increases the auxiliary power source voltage, the power increased by the voltage step-up circuit may be still insufficient for performing an emergency report due to the operation loss in the voltage step-up circuit. As a result, the device may fail to properly carry out the emergency report. As described above, in the conventional configuration, there exists a difficulty in effective use of the auxiliary power source.